1. Field of the Invention
The invention is generally directed to an adhesion promoting layer for bonding polymeric adhesives to metal and, more particularly, a heat sink assembly using the adhesion promoting layer as a thin adherent interfacial bonding layer between a multi-chip module cap and a polymeric adhesive used for attachment of a heat sink.
2. Description of the Related Art
Electronic devices such as computers rely on a large number of integrated circuits and other electronic components for their operation, most of which are mounted on printed circuit boards. Many of these components generate heat during normal operation. In order to accommodate increased demands for high operational speeds, a multi-chip module (xe2x80x9cMCMxe2x80x9d) system has been developed and put into practical use. An MCM mounts a plurality of integrated circuit (IC) chips next to each other on a single substrate. This MCM arrangement can permit high density assembly and effectively reduces the lengths of connecting wires to endow the electronic circuit with increased operational speed.
However, as operational speed requirements increase, the amount of heat that the components must dissipate generally also will increase. Many components need the help of external heat sinks to dissipate the operational heat generated. The term xe2x80x9cheat sinkxe2x80x9d, as used herein, generally refers to a passive device, for example, an extruded aluminum plate with a plurality of fins or a water cooled cold plate, that is thermally coupled to an electronic component(s) to absorb heat from the component(s) and dissipate such absorbed heat into the air or water by convection. While the heat sink itself generally is passive in nature, it is to be understood that it is within the scope of this terminology to optionally direct a fan means at or attach a fan means to the heat sink fins to increase the rate of convective heat transfer, if desired, such as described in U.S. Pat. No. 5,396,403 (Patel).
As a matter of practical necessity, many components often will share a common heat sink where high packing density and/or small individual component sizes are involved. One approach to providing such a shared heat sink is described in U.S. Pat. No. 5,396,403 (Patel), mentioned above, which relates to a cooling structure for high power MCM systems using flip chip technology. In the Patel patent, the primary heat path is for heat to dissipate, in the following sequence, through the metallized back sides of high power chips having the heat generating components mounted on the opposite chip sides, through an interface of indium solder, through a thermally conductive plate of silicon carbide or a tungsten copper composite, through an interface of thermal paste, and lastly through a heat sink such as made of extruded aluminum. The heat sink described by the Patel patent is an integral structure which both constitutes a housing for the MCM and it is finned on its exterior to dissipate the heat into the surrounding air by convection. Yet, the need to form and align four separate and distinct thermal intermediary layers, mentioned above, in-between the chips and the heat sink without corrupting the device represents a relatively complicated and intensive manufacturing scheme for attaching the heat sink to the chips.
Another proposal, described in U.S. Pat. No. 5,387,815 (Nishiguchi), relates to a structure for cooling one or more high power flip chips in a semiconductor module. The stated objective of the Nishiguchi patent is to provide an excellent heat dissipation design with a small number of components. The Nishiguchi patent describes a semiconductor chip module having a substrate on which a wiring portion is formed, a semiconductor chip mounted so as to face a circuit side down to the wiring portion, a heat sink with an end in contact with a side opposite to the circuit side of the semiconductor chip, and a cap enclosing the semiconductor chip and having an opening exposing externally the other end of the heat sink. The Nishiguchi patent describes a metal film having a wetting property with respect to the solder to be used as being formed on the inner wall of the cap opening and on the tip and/or side surface of the heat sink which is inserted into the cap. An adhesive solder material is filled in the gap between the tip portion of the heat sink and the back side of the semiconductor chip, and adhesive solder material is also filled between the metal films on the inner wall of the cap opening and the side surface of the heat sink to hermetically seal the cap. However, the time and resources needed to be devoted to forming and precisely aligning cap openings and heat sinks inserted therethrough with chip surfaces, as well as concerns for device corruption due to solder overflow and the needed additional precautions taken in this respect, will restrict yield and reliability.
As another approach, a heat sink for thermal cooling has been attached with a filled silicone elastomer adhesive to the upper exterior side of a flat-topped MCM cap. The silicone elastomer adhesive used in this regard has been a silicone potting resin conventionally used in the electronic industry for encapsulating electronic assemblies and devices (e.g., the xe2x80x9cSylgardxe2x80x9d trademarked series of silicone resins, made by Dow-Corning). The inner surface of this flat-topped MCM cap thermally communicates with chips housed in the cap via a thermal paste. Attachment of the heat sink to the outside of the MCM cap with the silicone resin has been accomplished directly to an anodized aluminum or a ceramic cap surface.
However, a problem arises when the attachment locus is a high performance MCM cap made of a highly thermal conductive material containing copper, such as copper tungsten (CuW). Usage of these copper-containing caps have become more relevant as thermal coefficient of expansion and thermal dissipation concerns change and evolve with ever increasing power needs. In order to attach a heat sink to the back surface of an MCM cap containing copper, such copper-containing caps have been metallized to provide and maintain a hermetically encapsulated package. A primary reason that such metallization is needed is attributable to the presence of an exposed continuous copper network in a composite such as copper tungsten, for example, which is susceptible to corrosion. The problem is heightened by the fact that contemporary MCM""s often must maintain operational capability for ten years or more. One such metallization technique involves nickel plating the entire surface of the cap. The nickel-plated cap is selectively gold plated on the seal band area of the cap so that a hermetic solder seal (e.g., Pb/Sn) could be provided between the cap and a substrate supporting the chips.
However, the direct adhesive bond between the nickel plating on the cap and the silicone resin is inadequate as the nickel and the silicone have compatibility problems. Consequently, the nickel-silicone bond is prone to delamination and thus fails to meet current package performance and reliability requirements. However, the resort to bonding the highly conductive cap to a separate, discrete heat sink structure via the nickel to silicone resin bond is difficult to design around, especially where a combination of low thermal coefficient of expansion and high thermal conductivity are desired. For example, high cooling requirements tend to require a large (tall) heat sink. It would be very difficult and very costly, if not impossible, to fabricate an integral cap and heat sink with highly conductive cap materials such as either copper tungsten (CuW) or aluminum silicon carbide (AlSiC). In the case of CuW, the additional weight would also constitute a large liability (e.g., WCu is about six times as dense as aluminum).
From a more general perspective, the use of a thin layer containing chromium, zinc, or preferably a mixture of chromium and zinc, to enhance the adhesion between a lead frame and a polymeric molding resin, has been described in U.S. Pat. No. 5,343,073 (Parthasarathi). A mixture of chromium and zinc with a zinc-to-chromium ratio in excess of about 4:1 is described as most preferred. The lead frame is formed from an electrically conductive metal substrate.
It is also known to use a tantalum, titanium, or chromium film to promote adhesion between a metal substrate and a fluorocarbon film. Also, U.S. Pat. No. 4,582,564 (Shanefield) describes a method of forming adherent metal layers on certain epoxy substrates by providing a thin metal film on surfaces of the epoxy substrates after the substrates are pre-conditioned by sputter etching to remove weak boundary layers from the surface, and then depositing primary metal films over the thin adhesion-promoting base metal film. The thin adherent metal films are formed by vacuum depositing an adherent thin metal film of chromium, nickel, a nickel-vanadium alloy, platinum, palladium, or titanium in thicknesses from 50 to 10,000 xc3x85ngstroms, generally 1,000 xc3x85ngstroms, onto an epoxy surface. The Shanefield patent characterizes the use of the vacuum-deposited adhesion-promoting films as appearing to be unique for only rubber-modified epoxy and epoxies having a high degree of unsaturation in the polymer chain.
From the foregoing it will be apparent that there remains a need for a way to effectively bond a silicone elastomer adhesive to a metal having low adherability to that adhesive, such as nickel,,and that there remains a particular need for a facile and less precision taxing approach to consolidate a heat sink assembly with an MCM in which a nickel-plated, highly thermal conductive MCM cap must be reliably bonded to a silicone elastomer, which, in turn, is used to join a heat sink to the cap.
It is an object of this invention to provide an adhesion promoting layer affording good bonding between polymeric adhesives and metal.
It is another object of this invention to provide an adhesion promoting layer providing high bonding efficacy between a silicon-containing polymeric adhesive and a metallic material otherwise having poor direct adherability thereto.
It is yet another object of this invention to provide a heat sink assembly for dissipating heat generated by chips in a multi-chip module (xe2x80x9cMCMxe2x80x9d) having means for providing a good reliable bond as between an MCM cap and a heat sink adhesive used to attach a heat sink to the MCM.
Briefly and in general terms, in one embodiment of the present invention there is a heat sink assembly including a thin adherent metal film provided between the metal-surfaced cap of an MCM and a polymeric adhesive layer used to attach a heat sink to the MCM, where the thin adherent metal film is a type of metallic material and of a thickness effective to bond the cap surface to the polymeric adhesive. The MCM is of the type involving a multi-chip module having a substrate and at least one integrated circuit chip mounted on a first surface of the substrate with the cap enclosing the integrated circuit chip(s), and where means is also provided for effecting thermal transmission between the integrated circuit chip(s) and the cap. The thin adherent metal film promotes bonding between the upper surface of the cap surface and the polymeric heat sink adhesive, and thereby provides an interfacial bond meeting package performance and reliability requirements.
The polymeric adhesive used to bond the heat sink to the MCM can be a thermoplastic, a thermosetting, or an elastomeric polymeric material having suitable heat sink adherability, water impermeability, resistance to swelling in hydrocarbon solvents, and the like attributes that are appropriate for an MCM environment.
In one advantageous embodiment of the invention, the aforesaid thin adherent metal film is employed to promote and provide adhesion between: (a) a metal surface of an MCM cap having poor direct adherability to silicon-containing polymeric adhesives, and (b) a silicon-containing polymeric adhesive (e.g., a silicone elastomeric material) being used to attach a heat sink to the MCM. For purposes of this invention, a metal having xe2x80x9cpoor direct adherabilityxe2x80x9d to silicon-containing polymeric adhesives means the metal has a shear strength to Sylgard(trademark) 6605 silicone elastomeric material (direct contact) of less than 150 lbs./inch2 (p.s.i.). Metals having such poor direct adherability to silicon-containing polymeric adhesives, such as silicone elastomeric materials in particular, include, for example, nickel, copper, silver, gold, and alloys thereof.
In one preferred embodiment of the present invention, the thin, interfacial adherent metal film is formed of a metal selected from Groups IVB, VB, VIB of the Periodic Table of the elements (CAS version), or alloys thereof, excluding the radioactive elements Unq, Unp, and Unh. Chromium, or an alloy of chromium, is the most preferred metal for the thin, interfacial adherent metal film used in this invention.
The thin adherent metal film used in the present invention is deposited on a metal surface, such as a nickel plated MCM cap, in a film thickness of from 50 xc3x85ngtroms to 10,000 xc3x85ngstroms, more preferably from about 450 xc3x85ngtroms to about 550 xc3x85ngstroms.
In another preferred embodiment of the invention, the MCM cap surface is nickel, such as a nickel plating formed on a cap substrate that would be susceptible to corrosion if left exposed (i.e., unplated). For instance, when the cap contains copper, such as in a copper tungsten composite, and the nickel-plating serves to prevent corrosion of the copper component of the cap.
The technique for depositing the thin adherent metal film on a metal surface, such as a metal having poor adherability to silicon-containing polymeric adhesives (e.g., a silicone elastomeric material), includes vacuum deposition techniques, such as sputter deposition, and also other ion plating deposition techniques such as electrolytic plating, which permit thin film formation. Sputter deposition is the preferred mode of depositing the thin adherent metal film on the metal surface.
The present invention offers many benefits and advantages. To name several, the present invention uses a facile yet effective metallization step to promote and provide adhesion between an MCM cap plated with a metal (e.g., nickel) otherwise having low direct adherability to silicon-containing polymeric adhesives, such as a silicone elastomeric material, and a silicon-containing polymeric adhesive being used to attach a heat sink to the cap. This inventive metallization step can be accomplished without adversely disturbing hermetic sealing metallurgy. For example, the metallization of a nickel-plated MCM cap can be accomplished without damaging or adversely affecting a metallized gold plated seal band used for hermetic sealing of the cap to a substrate. Further, the heat sink assembly of this invention can fully withstand temperature cycling, T/H and shock exposures expected in the operating environment of an MCM device over extended periods of time and operation. The invention also reduces the cost of otherwise mounting a heat sink with mechanical hardware, and it minimizes space requirements by eliminating mounting hardware. Also, the invention affords manufacturing flexibility as to the timing of the metallization of a nickel-plated MCM cap. For example, the nickel-plated MCM cap surface can be pre-coated with the thin adherent metal film that promotes adhesion to silicon-containing polymeric adhesives, for instance, before attachment of the cap to the substrate and integrated circuit chips, thereby eliminating risk of module yield loss. The invention also provides a heat sink assembly in combination with an MCM system having enhanced tolerance to combined stresses of high temperature, high humidity, and/or high pressure, as demonstrated by a highly accelerated stress test (i.e., xe2x80x9cHASTxe2x80x9d). The highly accelerated stress test refers to putting parts to be tested in a sealed chamber where they are simultaneously subjected to high temperature (125xc2x0 C.), high humidity (85%), and high pressure (about 3 atmospheres absolute), for 24 hours. In this manner, stress equivalent to the life of the product can take place in an accelerated number of hours.
In this invention, the terminology xe2x80x9cpolymeric adhesive(s)xe2x80x9d means a natural or synthetic adhesive substance containing polymers.
For purposes of this invention, the terminology xe2x80x9csilicon-containing polymeric adhesive(s)xe2x80x9d means an adhesive substance containing polymeric molecules each having at least one silicon atom in any of the backbone, pendant groups and/or terminal groups of the polymeric chain.
In this invention, the terminology xe2x80x9csilicon-containing elastomeric materialxe2x80x9d means a silicon-containing polymeric adhesive that is elastomeric. The term xe2x80x9celastomericxe2x80x9d refers to polymers that are capable of significant elastic deformations.
In this invention, the terminology xe2x80x9csilicone elastomeric materialxe2x80x9d means a silicon-containing elastomeric material that is a composition containing silicone polymers, where the silicon polymers are each based on a polymeric chain featuring alternating silicon (Si) and oxygen (O) atoms in the backbone, and where the backbone is devoid of or substantially devoid of carbon atoms excluding consideration of terminal end groups, and the silicon polymers also have organic side groups attached to silicon atoms located in the backbone.